It is known to form micron-sized channels in substrates, such as microscope slides, for the purpose of performing a variety of chemical measurements. These substrates, with patterned channels, have come to be known as "microchips."
To date, micromachined fluid pumps have had limited performance and are not appropriate for many microchip applications. Other mechanisms of material transport, such as electrokinetic phenomena, i.e., electroosmosis and electrophoretic transport, are of interest due to the ease with which they can be incorporated into microfabricated devices.
With microchip devices, several applications require the ability to precisely spatially confine a sample with consistent reproducibility. To this end, related co-pending application Ser. No. 08/283,769 filed Aug. 1, 1994 provides a structure for electrically providing material transport through microchip structures with very good spatial confinement.
Examples of microchip devices and methods can be found in the following: U.S. Pat. No. 5,296,114 to Manz; "Micromachining a Miniaturized Capillary Electrophoresis-Based Chemical Analysis System on a Chip," by D. J. Harrison et al., Science, Vol. 261 (Aug. 13, 1993); "Glass Chips for High-Speed Capillary Electrophoresis Separations with Submicrometer Plate Heights," by C. S. Effenhauser et al., Anal. Chem., 65, 2637-2642 (1993); "Effects of Injection Schemes and Column Geometry on the Performance of Microchip Electrophoresis Devices," by S. C. Jacobson et al., Anal. Chem., 66,1107-1113 (1994); and "Open Channel Electrochromatography on a Microchip," by S. C. Jacobson et al., Anal. Chem., 66, 2369-2373 (1994).
While the foregoing publications describe general techniques and devices for performing fluidic manipulations, a continuing need exists for improvements in "electrodynamic focusing." Electrodynamic focusing refers to the use of electrokinetic transport to confine spatially the transport of both fluids and ions. Electrodynamic focusing differs from hydrodynamic focusing which uses pressure driven flow to confine the sample stream. Electrodynamic focusing includes the forces from electroosmotic fluid flow and electrophoretic forces due to electrostatic fields. In the context of this invention, sample stream implies the transport of a sample material either by electroosmotic flow, electrophoretic motion, or both.